This invention relates generally to chopper stabilized amplifiers, and in particular to a chopper stabilized amplifier having an additional differential amplifier stage for improved noise reduction.
Complementary metal-oxide-semiconductor (CMOS) technology is popular in forming integrated circuit field-effect transistor (FET) operational amplifiers. A problem in creating differential amplifiers using CMOS technology, however, is keeping the input offset voltage both small, e.g., less than one hundred microvolts, and very stable over time and temperature. The offset voltage of CMOS amplifiers is caused by systematic bias errors between stages and mismatches between identical MOS devices.
Chopper stabilized amplifiers are well known in the electronics field, and have been used for many years for designing precision DC amplifiers. Traditionally, they achieve a high degree of DC stability by "chopping" or modulating the input DC signal with a high-frequency clock to produce an AC signal, amplifying the modulated AC signal as well as any amplifier offset signal, and then synchronously demodulating the AC signal back into a DC signal. Any input offset signal remains modulated at the chopping frequency, and is filtered.
One of the drawbacks to the foregoing approach is that the amplifier is essentially a sampled data system, the bandwidth of which is limited to one-half the Nyquist sampling frequency (the chopping frequency). To overcome this drawback and provide wide bandwidth amplifying characteristics, a fast AC-coupled operational amplifier was added to the chopper-stabilized amplifier to provide a composite amplifier structure. The chopper-stabilized amplifier biases the fast operational amplifier's non-inverting input to force the summing point to zero. The high-frequency roll-off of the chopper-stabilized amplifier must coincide with the low-frequency cutoff the fast AC-coupled amplifier in order for the amplifier to have smooth gain and phase characteristics.
Conventional CMOS chopper-stabilized operational amplifiers as described above are commercially available from a number of vendors. The fast AC-coupled amplifier is characterized as the main amplifier, and chopper-stabilized amplifier is characterized as the nulling amplifier. The main amplifier is connected full time from the input to the output, while the nulling amplifier operates under control of a chopper clock to alternately null itself and the main amplifier. It is in the switching of the output of the nulling amplifier to main amplifier's non-inverting input that a slight mismatch in voltages contributes unwanted noise into the output of the main amplifier.